The present document pertains to technology performing radio communications.
Recently, technologies in which no communications infrastructure such as radio base stations is present and in which communications are carried out between radio terminals only (ad-hoc networks) has been studied. Regarding ad-hoc networks, specification investigations and standardization are carried out centered on the MANET (Mobile Ad-hoc NETworks) Working Group of the IETF (Internet Engineering Task Force), there being not only direct communications between mobile terminals (single hop) but also communications carried out via one or several other terminals (multihop).
However, in case a mobile terminal leaves the range of the whole network due to movement or the like, there exists an interval during which communication becomes impossible. As against this, there are technologies in which, when data are transferred from a source terminal transmitting data (hereinafter called the “transmission source terminal”) to the destination terminal to which data are sent and delivered (hereinafter called the “destination terminal”), a terminal A (below called the “relay terminal”) to which data have been distributed from the transmission source terminal and which relays data, searches for a relay terminal B to which the data should next be distributed, distributes the data, and the data are sent and delivered all the way to the destination terminal while relaying data among terminals.
Also, there are electronic mail systems performing data transfers to specific terminals. In an electronic mail system, the transmission source terminal specifies the destination terminal. Next, the transmission source terminal performs data transfer taking another terminal to be a relay terminal, on the basis of network topology information and the like possessed in advance, and subsequently requests data transfer to the concerned relay terminal. There are also cases of carrying out an error report to the transmission source terminal or the transferring relay terminal in case the data transfer is unsuccessful. Moreover, there are also cases where the destination terminal receiving the data reports an acknowledgment that the data have been received to the transmission source terminal.